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Emanuela Mattioli, Fabienne Giraud, Bernard Pittet:
The contribution of calcareous nannoplankton to the carbonate production in the Jurassic
(Talk)

In the Jurassic, the main pelagic organisms to produce a calcareous test are coccolithophorids and the incertae sedis Schizosphaerella spp. Quantification of their productivity is therefore a valid tool to understand production and transfer of pelagic carbonates to the sedimentary reservoir. In order to understand the contribution of pelagic carbonate production to the sedimentary record, numerous sections have been studied in the Toarcian, a period of overall decreased carbonate productivity, and in the Late Oxfordian-Early Kimmeridgian, which is on the contrary a period of enhanced carbonate productivity. Different palaeolatitudinal settings have been examined: Toarcian sections are from central Italy and NW France, the Oxfordian-Kimmeridgian interval has been studied in SE and NE France and S Germany. Proximal and more-distal sections, with respect to carbonate platforms, have been sampled.

Some recurring features have been observed in all the studied sections, independent of age and latitude: 1) nannofossil abundances decrease with increasing carbonate content; 2) the highest abundances of nannofossils are found in condensed levels; 3) Schizosphaerella abundance, higher in proximity of carbonate shallow platforms, first increases with increasing CaCO₃ content (until around 60%), then decreases; and 4) Schizosphaerella size covaries with respect to carbonate content. Although diagenetic overprinting has affected the nannofossil record in the studied lithotypes, diagenesis is not thought to be the main factor controlling nannoplankton abundance. Due to the long residence time at the water/sediment interface in periods of low sedimentary rates, severe diagenetic effects would be expected on nannofossils. However, condensed levels encountered in different sections display various degrees of nannofossil preservation but commonly exhibit the highest nannofossil record.

Such observations provide evidence that: a) nannofossils did not produce the bulk of carbonate mud in the Jurassic, although they contribute, to different extents, to the pelagic fraction; b) the contribution of Schizosphaerella, being much bigger and often more abundant than coccoliths, to carbonate production was higher; and c) the contribution of both coccoliths and Schizosphaerella to carbonate production became important only in periods of reduced accumulation rates.

Two main factors have likely influenced the carbonate record in hemipelagic environments during the Jurassic, namely nannoplankton productivity (with a dominance of Schizosphaerella) and, mainly, dilution cycles by imported carbonate mud (probably of platform origin).

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